An uninterruptible power supply (sometimes referred to as an "uninterruptible power system" but in both cases generally abbreviated to "UPS") is distinct from a standby power supply. In the event of a fault condition an UPS can maintain the supply electrical power to a critical load with no break and no perceptible discontinuity at the time of the fault. Such power supplies are essential for large computers where even momentary loss of power below the industry guideline of 16% below nominal level may cause a system shut-down at a very considerable cost. Furthermore under some conditions data may be lost and may be almost impossible to recreate thereby causing concern for financial transactions and other areas where security is important. Such supplies are therefore intrinsically different to a standby or back-up power supplies where a disruption of a few seconds before the alternate source can commence supply is not really critical. An ideal UPS would includes means to test itself and particularly whether its batteries are still capable of delivering full current, while under normal load, without interruption of the supply to its critical load.